Development of an analytical methodology for obtaining quantitative mass concentrations from LAAP-ToF-MS measurements

Abstract

Laser ablation aerosol particle-time of flight mass spectrometer (LAAP-ToF-MS) measures the size number of particles, and chemical composition of individual particles in real-time. LAAP-ToF-MS measurements of chemical composition are difficult to quantify, mostly because the instrument sensitivities to various chemical species in the multicomponent atmospheric aerosol particles are unknown. In this study, we investigate a field-based approach for quantitative measurements of ammonium, nitrate, sulfate, OC, and EC, in size-segregated atmospheric aerosols, by LAAP-ToF-MS using concurrent measurements from high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), and multi-angle absorption photometer (MAAP). An optical particle counter (OPC) and a high-resolution nanoparticle sizer (scanning mobility particle sizer, or SMPS), were used to measure the particle size distributions of the particles in order to correct the number concentrations. The intercomparison reveals that the degree of agreement of the mass concentrations of each compound measured with LAAP-ToF-MS and HR-ToF-AMS/MAAP increases in the following order NH < SO < NO < EC < OC < Cl with r values in the range of 0.4-0.95 and linear regression slopes ranging between 0.62 and 1.2. The factors that affect the mass concentrations measured by LAAP-ToF-MS are also discussed in details. Yet, the matrix effect remains one of the strongest limiting factor to achieve an absolute quantification of the aerosol chemical composition. In the future we suggest the development of a methodology based on the calculation of the response factors generated by different types of particles, which could possibly resolve certain difficulties associated with the matrix effect.

A new method to quantify mineral dust and other aerosol species from aircraft platforms using single particle mass spectrometry

“…Fortunately, the real refractive indices for background tropospheric environments have been observed to fall within a narrow range (n ≈ 1.50 -1.56 at mid-visible wavelengths) such that ambient particle size measurements are not strongly affected by this assumption (Hand and Kreidenweis, 2002;Liu et al, 2008;Müller, 2002;Reed Espinosa et al, 2017;Shingler et al, 2016;Yamasoe et al, 1998). Ammonium sulfate and ammonium nitrate (n=1.53 and 1.56 at λ=532 nm, respectively) are common inorganic constituents.…”

“…Relative abundance measurements of internally mixed aerosol sub-components have been reported in PALMS and other SPMS systems for metals (Cziczo et al, 2001;Murphy et al, 2007;Zawadowicz et al, 2015), organosulfate species (Froyd et al, 2010;Liao et al, 2015), elemental carbon (Healy et al, 2012), and sulfate and organic material 25 (Jeong et al, 2011;Middlebrook et al, 1998;Murphy et al, 2006;Zelenyuk et al, 2008;Zhou et al, 2016). Some groups have scaled SPMS data rates to aerosol reference instruments to derive total number or mass concentrations (Bein et al, 2006;Pratt et al, 2009;Qin et al, 2006) or concentrations for specific particle types (Gemayel et al, 2017;Healy et al, 2012;Jeong et al, 2011;Reinard et al, 2007;Shen et al, 2018). Many of these scaling studies invoke potentially large assumptions such as constant SPMS detection efficiencies or a single density applied to all particles that can strongly affect derived concentrations.…”

Understanding of atmospheric aerosol particles with improved particle identification and quantification by single particle mass spectrometry

“…Such an uncertainty is difficult to reduce, since we don't have particle shape information for individual particles. It should be noted that in previous studies, the particle shapes were also assumed as spherical, and averaged particle densities (~1.6 to 1.9 g cm -3 ) based on the comparison between d va and d m were applied for total aerosol particle mass quantification (Gemayel et al, 2017;Jeong et al, 2011;Zhou et al, 2016). The density for different types of ambient particles varies, which will be shown in the following text.…”

“…An even more challenging issue is the quantitative analysis of individual particles' mass and chemical composition, which cannot be directly provided by SPMS measurements, because laser ablation only allows an a priori unknown fraction of the single particle to be vaporized and ionized (Murphy, 2007). In addition, matrix effects may obscure the particle composition (Gemayel et al, 2017;Gross et al, 2000). Our previous laboratory SPMS study also verified the difficulty of particle quantification due to incomplete ionization, which could not be improved significantly by replacing the originally used nanosecond excimer laser with a femtosecond laser with higher laser power density and shorter laser pulse length (Ramisetty et al, 2018).…”

“…This scaling approach was improved by applying a hit rate correction (Zhou et al, 2016). Combining AMS, optical particle counter (OPC), and SPMS data, Gemayel et al (2017) also quantified the fragments aforementioned in size-segregated atmospheric aerosols measured by SPMS in the field. In addition, Ahern et al (2016) used the peak area ratio of organic matter marker at m/z 28 CO + to EC markers (C 2 -5 + ) to account for laser shot-toshot variability, and demonstrated a linear relationship between normalized organic intensity and secondary organic aerosol (SOA) coating thickness on soot particles.…”